1. Field of the Invention
The present invention relates generally to an optical coupler circuit, and, in particular, to a gain compensation circuit for an optical coupler.
2. Description of the Related Art
Optical coupler circuits, also referred to as optically coupled isolator devices or optocouplers, provide isolation between different circuit portions which, for example, operate at vastly different voltages. An optocoupler typically includes a light source, such as an LED, at one side and a light sensor, such as a phototransistor or a photodiode, at the other side. Changing light levels from the LED traverses a gap and are received by the light sensor. The imaginary division between the circuit portions operating at the two different voltages is referred to as an isolation barrier and the optocoupler communicates across this isolation barrier. The current gain of an optical coupler between the input and the output is referred to as the current transfer ratio (CTR) of the optical coupler. The CTR is the ratio of the output current which is the same as the collector current to the input current which is the current exciting the LED.
Optocouplers are commonly used in switched-mode power supplies and other analog circuits to provide an analog feedback control signal across the isolation barrier. Optocouplers are used because they are small, simple, easy to use, and reliable. On the other hand, optical couplers have large initial gain variations. The initial gain variation can occur from device to device or from production lot to production lot in typical optical couplers. The CTR range variation introduces undesirable variations in performance and capability of a circuit that must transmit information across the isolation barrier.
A potential solution to this problem is to actively trim a gain setting resistor to null out the initial CTR (Current Transfer Ratio) variations. A typical optocoupler feedback circuit has an overall circuit gain that is proportional to the current transfer ratio of the optocoupler divided by the impedance of the setting resistor. Thus, varying the resistor impedance value will adjust for variations in the optocoupler gain.
An example of a circuit utilizing such active trimming is shown in FIG. 1, including a power supply 10 with an optocoupler feedback circuit 12. The power supply 10 is illustrated only generally for purposes of simplicity and includes an input 14 for an input voltage Vin, a power transformer 16, and a rectifier and filter 18 at the secondary side of the transformer 16. A DC output voltage Vout is produced at the output of the rectifier and filter 18 and is available at 20.
The high and low voltage portions of the power supply are separated by an isolation barrier 22, an imaginary boundary indicated in the drawing by a line. An optocoupler 24 is used to communicate a feedback signal across the isolation barrier 22 so that the output voltage Vout of the power supply can be precisely regulated. In the feedback circuit, the optocoupler 24 is driven from an error amplifier circuit which includes the light emitting diode 26 of the optocoupler 24, an npn transistor 28, and a trim resistor 30, as well as a operational amplifier 32 driving the base of the transistor 28. The operational amplifier 32 receives the output voltage Vout through a voltage divider of resistors 34 and 36 at the non-inverting input 38, while a reference voltage Vref at 40 is fed through a resistor 42 to the inverting input 44. The inverting input 44 of the operational amplifier 32 also receives the voltage across the trim resistor 30 via a capacitor 46. A test point 48 is provided at which the voltage across the trim resister 30 is measured.
On the input side of the circuit, on the other side of the isolation barrier 22, a phototransistor or photodiode 50 of the optocoupler 24 is connected to a second trim resistor 52 as well as to the input of an error amplifier, pulse width modulator and driver circuit 54. The circuit 54, while having multiple functions, is available on a single chip. The output of the error amplifier, pulse width modulator and driver circuit 54 is fed to the power transformer 16, potentially through a switch (not shown). An input test point 56 is utilized to measure the voltage across the trim resistor 52.
The error amplifier compares the output voltage 20 against the reference voltage 40 in the operational amplifier 32 and produces an error voltage at 58 to provide closed loop regulation of the power supply. The error voltage 58 is converted to a current by the transistor 28 and the trim resistor 30. This current is transferred across the isolation barrier 22 by the optocoupler 24, where it is converted back to a voltage at the trim resistor 52.
The voltage gain of the optocoupler circuit 12 is determined from the ratio of the test point 48 to the test point 56. The test point 48 is the feedback error voltage and the point 56 is the pulse width modulation control voltage of the circuit 54. The voltage gain Av is determined from the equation
      Av    =                  V56        V48            =                        CTR          ·          R52                R30              ,wherein V56 is the voltage at test point 56, V48 is the voltage at test point 48, R52 is the resistance of the resistor 52, R30 is the resistance of the resistor 30 and CTR is the current transfer ratio of the optocoupler 24.
The circuit of FIG. 1 is used with an active laser trimming process to eliminate performance sensitivity to initial gain (CTR) variations associated with the optocoupler circuit. The active laser trimming process adjusts the resistors 30 and 52 depending on the initial current transfer ratio of the optocoupler 24. By trimming the resistor 52 to a higher resistance, an adjustment is made to increase the gain for low current transfer ratio optocouplers, and by trimming the resistor 30 to a higher resistance, an adjustment is made in the gain for high current transfer ratio optocouplers. Thus, the voltage gain of the circuit is set to the desired level by compensating for initial variations in the current gain (CTR) of the optocoupler.
However, the current transfer ratio of the optocoupler increases or decreases with changes in temperature, and it also suffers a degradation over the life of the device. Over the typical life of an optocoupler, a loss of 30 to 50 percent of the gain can be expected. The drivers for optocouplers permit feedback loop crossover frequencies to vary as the gain of the optocoupler varies. Lower performance from the isolated feedback loop results, along with a degraded performance of the optocoupler. No adjustment is made for changes in the current transfer ratio due to changes resulting from temperature variations or aging. Instead, a margin for error for these changes is built into the circuit. Even in light of this, the gain of the feedback circuit changes with the age and temperature.
Referring to FIG. 4, an embodiment of an optocoupler feedback circuit is shown without the power supply. The illustrated circuit includes the optical coupler 24 connected to a shunt regulator 100. The shunt regulator 100 regulates the operating current of the optocoupler 24 as necessary to keep the output voltage Vout constant. As the output voltage Vout increases, the current through the shunt regulator 100 and through the light emitting diode 26 of the optocoupler 24 increases, thereby producing a larger feedback signal at the test point 56. A capacitor 101 may be connected between the control lead and the cathode of the shunt regulator 100.
A control circuit, such as shown in FIG. 1, attached at the test point 56 lowers the power supply duty cycle. This causes a slight decrease in the output voltage Vout and keeps the output voltage in regulation. The output voltage Vout is connected at 102 to a voltage divider made up of resistors 104 and 106, the midpoint of which is connected to the control lead of the shunt regulator 100.
A resistor 108 is between the optocoupler 24 and the power supply voltage at 110, which in this case is the output voltage Vout. The low voltage test point 48 is between the resistor 108 and the optocoupler 24. On the other side of the isolation barrier 22, the light sensor 50 of the optocoupler 24 is connected to the resistor 52 to define the voltage at the test point 56.
The voltage gain in the circuit illustrated in FIG. 4 from the test point 48 to the test point 56 is
      Av    =                  V56                  Vout          -          V48                    =                        R52          ·          CTR                R108              ,wherein Vout is the voltage at 102 and 110, V56 is the voltage at the test point 56, V48 is the voltage at the test point 48, R52 is the resistance of the resistor 52, R108 is the resistance of the resistor 108, and CTR is the current transfer ratio of the optocoupler 24.
Typically, in feedback loops with a shunt regulator 100 as the voltage regulating device, the current transfer ratio of the optocoupler 24 is not compensated. The current transfer ratio rating of the optical coupler 24 is incorporated into the circuit. However, this results in feedback loop variations that yield a gain variation in the circuit of 10 to 20 dB, and a corresponding feedback loop frequency crossover that varies by as much as a factor of 10.